This invention relates to a method and apparatus for measuring the thickness of a workpiece by the utilization of ultrasonic pulses in which ultrasonic pulses are periodically transmitted into the workpiece from one surface thereof so that ultrasonic echo pulses reflected from the bottom surface of the workpiece are received, and in which the period of time elapsed from the time of transmission of each ultrasonic pulse to the time of reception of its related echo pulse is measured to determine the thickness of the workpiece.
In carrying out the above method, the measurement of the elapsed time period is generally performed by counting the elapsed time period by clock pulses, and, therefore, the resolution or accuracy in the thickness value finally obtained depends on the frequency or period of the clock pulses.
Recently, user's requirements for improved resolution of ultrasonic thickness measuring apparatus have become severe, say, the resolution of 0.1 mm or less is required under the condition that the sound velocity within a workpiece is about 10,000 m/sec or more. In the case where the sound velocity is set at 10,000 m/sec and the resolution of 0.1 mm is required, it is necessary to use clock pulses having the period of 2.times.10.sup.-8 sec (20 nano-seconds), that is, clock pulses having the frequency of as high as 50 MHz.
However, at present, a semiconductor IC oscillator having such a high upper frequency limit as 50 MHz is very expensive and involves consumption of a large amount of electric power. Thus, the known ultrasonic thickness measuring method and apparatus have the defect that they not only require high expenditure but also involve consumption of a large amount of electric power.